Immunopathologic studies will be undertaken to define the nature of the severe anemia that occurs in the course of babesial infection. Turnover and uptake studies of purified and radiotagged rat C3 will be done to determine the nature of the complement-dependent anemia. The putative immune complexes circulating in the blood during infection will be further examined in order to define the nature of the immune antigen and the antibody constituting these complexes.